The present invention relates to a light emitting diode (LED) having a double hetero-structure and more particularly to an LED that has a high optical power and can be used at a large current.
A highly efficient LED having a double hetero-structure as shown in FIG. 21 is known. FIG. 21 is a vertical sectional view showing an AlGaInP LED in which layers are lattice-matched with a GaAs substrate 1. The structure of each layer in the LED is as follows:
Substrate 1:
made of an n-type GaAs
Buffer layer 2:
made of n-type GaAs
N-type cladding layer 3
made of n-type (Ga0.3Al0.7)0.5In0.5P
impurity: Si, impurity concentration: 1xc3x971018 cmxe2x88x923, and
thickness: 1 xcexcm
Light-emitting layer 4:
made of p-type (Ga0.7Al0.3)0.5In0.5P
thickness: 0.5 xcexcm
P-type cladding layer 5:
made of p-type Al0.5In0.5P
impurity: Zn, impurity concentration: 5xc3x971017 cmxe2x88x923, and
thickness: 1 xcexcm
First current diffusion layer 7:
made of p-type Ga0.3Al0.7As
impurity: Zn, impurity concentration: 1xc3x971018 cmxe2x88x923, and
thickness: 1 xcexcm
Second current diffusion layer 8:
made of p-type Ga0.3Al0.7As
impurity: Zn, impurity concentration: 3xc3x971018 cmxe2x88x923, and
thickness: 6 xcexcm
Contact layer 9:
made of p-type GaAs
An n-side electrode 10 is formed on the underside of the n-type GaAs substrate 1. A p-side electrode 11 is formed on the p-type GaAs contact layer 9.
The n-type GaAs buffer layer 2 is intended to eliminate defects of the n-type GaAs substrate 1 and influence of contaminants in the substrate and is not required if the n-type GaAs substrate 1 is surface-treated favorably. The p-type GaAs contact layer 9 has a GaAs structure not containing Al to facilitate an ohmic contact between the p-type GaAs contact layer 9 and the p-side electrode 11. The GaAs composing the contact layer 9 does not transmit light generated from the p-type (Ga0.7Al0.3)0.5In0.5P light-emitting layer 4, but no problem is raised because the contact layer is formed immediately below the p-side electrode 11.
Sharp K. K. has recently proposed an LED, a vertical sectional view of which is shown in FIG. 22, to achieve higher reliability than the above LED (Japanese Patent Application No. 10-338656). The structure of each layer in the LED is as follows:
Substrate 21:
made of n-type GaAs
Buffer layer 22:
made of n-type GaAs
N-type cladding layer 23:
made of n-type (Ga0.3Al0.7)0.5In0.5P
impurity: Si, impurity concentration: 1xc3x971018 cmxe2x88x923, and
thickness: 1 xcexcm
Light-emitting layer 24:
made of p-type (Ga0.7Al0.3)0.5In0.5P
thickness: 0.5 xcexcm
First p-type cladding layer 26:
p-type (Ga0.5Al0.5)0.5In0.5P
impurity: Zn, impurity concentration: 1xc3x971017 cmxe2x88x923, and
thickness: 0.2 xcexcm
Second p-type cladding layer 27:
made of p-type Al0.5In0.5P
impurity: Zn, impurity concentration: 5xc3x971017 cmxe2x88x923, and
thickness: 1.0 xcexcm
Current diffusion layer 28:
made of p-type Ga0.9In0.1P
impurity: Zn, impurity concentration: 1xc3x971018 cmxe2x88x923, and
thickness: 7 xcexcm
Contact layer 29:
made of p-type Gals
An n-side electrode 30 is formed on the underside of the n-type GaAs substrate 21. A p-side electrode 31 is formed on the p-type GaAs contact layer 29.
A p-type cladding layer 25 is formed as a two-layer structure consisting of the p-type (Ga0.5Al0.5),0.5In0.5P first cladding layer 26 and the p-type Al0.5In0.5P second cladding layer 27. Accordingly, it is possible to prevent a p-type impurity from diffusing to the p-type (Ga0.7Al0.3)0.5In0.5P light-emitting layer 24 although the p-type impurity has a large impurity gradient and is liable to diffuse when electric current flows through the LED for a long time. Thus it is possible to prevent deterioration of the optical power of the LED.
The LED is used in the form of a chip. Conventionally, an LED wafer is divided into chips of a size of 200 xcexcm-300 xcexcm by 200 xcexcm-300 xcexcm. In the above LEDs, the p-type GaAs contact layers 9, 29 and the p-side electrodes 11, 31 are formed circular and disposed at the center of each chip. FIG. 23 shows a planar configuration of the chip.
The above LEDs have the following problem: Electric current flows immediately below the p-side electrodes 11, 31. Both the p-side electrodes 11, 31 and the p-type Gabs contact layers 9, 29 disposed under the p-side electrodes 11, 31 are opaque. Thus, the p-side electrodes 11, 31 and the contact layers 9, 29 intercept light coming from parts of the p-type (Ga0.7Al0.3)0.5In0.5P light-emitting layers coming from parts of the p-type (Ga0.7Al0.3)0.5In0.5P light-emitting layers 4, 24 that are located immediately below the p-side electrodes 11, 31. Thus, the light coming from those parts cannot be taken out to the outside. Accordingly, the above LEDs have a low light-emitting efficiency.
The LED chips are conventionally used at electric current having an intensity of several milliamperes to 50 mA. If the LED chip is used for an electric current having intensity higher than that, the optical power of the LED chip will saturate and characteristics will deteriorate with the passage of a current.
Therefore, it is an object of the present invention to provide an LED in which light emission immediately below an electrode is restricted to improve light take-out efficiency so that the LED has an improved light-emitting characteristic when it is used at a large current of several milliamperes to 50 mA or more.
In order to accomplish the above object, there is provided, according to an aspect of the invention, a light emitting diode of a double hetero-junction type comprising:
a light-emitting layer composed of a GaAlInP material;
a p-type cladding layer and an n-type cladding layer sandwiching the light-emitting layer therebetween;
a p-side electrode formed on the p-type cladding layer side; and
an n-side electrode formed on the n-type cladding layer side;
the p-type cladding layer consisting of a first p-type cladding layer positioned closer to the light-emitting layer and having a lower aluminum content and a lower impurity concentration, and a second p-type cladding layer positioned farther from the light-emitting layer and having a higher aluminum content and a higher impurity concentration; and
a current blocking layer for locally blocking electric current flowing from the p-side electrode to the n-side electrode.
The current blocking layer may be provided immediately below the p-side electrode which is opaque. With this arrangement, electric current flowing to those parts of the light-emitting layer that are positioned immediately below the p-side electrode is restricted. By thus suppressing emission of unrequired light which would be intercepted by the opaque p-side electrode, it is possible to enhance the light take-out efficiency and thus improve the optical power. That is, the external light emission efficiency can be enhanced.
If the thickness of the first p-type cladding layer is within a range of 0.2 xcexcm to 0.5 xcexcm inclusive, an initial luminous intensity ratio of 100% can be obtained. Thus, it is possible to increase reliability of the LED.
In one embodiment, the p-side electrode has an electrode window consisting of an opening, and the current blocking layer has an opening at a position confronting the electrode window of the p-side electrode, and the opening of the current blocking layer serves as a current path for intensively passing electric current from the p-side electrode through the light emitting diode.
According to the structure, the current density is increased and thus the internal light-emitting efficiency is also increased. There is a fear that the increase of the current density will reduce the optical power if electric current is passed through the LED for a long time. But such reduction of the optical power can be suppressed because the p-type cladding layer consists of the first and second layers.
An appropriate current density can be obtained by setting the area of the current path to the range of 1,000 xcexcm2 to 40,000 xcexcm2. Consequently, the internal light-emitting efficiency can be enhanced. In the case where the area of the current path is set to the range of 1,000 xcexcm2 to 20,000 m2, a comparatively dark portion is prevented from taking place in the center of the current path even when the diameter of the current path is 150 xcexcm or more. In the case where the area of the current path is set to the range of 1,000 xcexcm2 to 10,000 xcexcm2, a high optical power can be obtained even when the LED is driven at 20 mA.
In one embodiment, the p-side electrode is formed at a central portion of a surface, and the current blocking layer is formed at a position confronting the p-side electrode such that electric current coming from the p-side electrode flows around of the current blocking layer.
The p-side electrode may be formed at a central part of a surface of a layer. In this case, the LED, which has a high optical power, is fabricated by using the same electrode-forming process as that conventionally used.
The current blocking layer may be formed inside a current diffusion layer. In this case, the current blocking layer is located in a position nearer to the light-emitting layer than when the current blocking layer is formed on the upper surface of the current diffusion layer. Accordingly, it is possible to prevent electric current whose path has been restricted by the current blocking layer from being unfavorably diffused before it reaches the light-emitting layer.
There is also provided, according to a second aspect of the present invention, a light emitting diode of a double hetero-junction type in which a light-emitting layer made of a GaAlInP material is interposed between a p-type cladding layer and an n-type cladding layer, wherein:
a p-side electrode is formed on a p-type cladding layer-side surface having an area of 0.15 mm2 or more; and
any point present in a region not containing the p-side electrode of the p-type cladding layer-side surface is within a distance of (Ldxc3x972) from some point on an edge of the p-side electrode, where Ld is a distance from a position at which an optical power is maximum, to a position at which the optical power attenuates by 90%.
According to the construction, it is possible to obtain a favorable current diffusion and thus, suppress the increase of the current density. Therefore, even if the LED is used at a large electric current, the current density will not become too high. Accordingly, it is possible to prevent saturation of the optical power of the LED and deterioration in application of electric current to the LED. Thus, it is possible to improve the light-emitting characteristic at a large current.
When the distribution of the optical output of an LED chip is examined along a line A-Axe2x80x2, as shown in FIG. 17A, passing through a p-side electrode 161, the distance Ld is a distance from a position close to the p-side electrode 161 at which the optical power is maximum, to a position at which the optical power attenuates by 90% as compared with the maximum value, as shown in FIG. 17B. Then, according to the present invention, as shown in an explanatory illustration of FIG. 18, the p-side electrode (denoted by 162 in FIG. 18) is provided such that any point present in a region not containing the p-side electrode of the p-type cladding layer-side surface is within the distance of (Ldxc3x972) from the edge of the p-side electrode.
The p-side electrode may comprise a plurality of branch electrodes and a connection electrode connecting the branch electrodes to each other electrically.
In one embodiment, an interval between the branch electrodes is approximately Ld.
The surface on which the p-side electrode is formed may have two opposed parallel straight sides, and the branch electrodes may be each strip-shaped, and arranged parallel with the two sides and with each other.
In one embodiment, an interval between an outermost branch electrode and the side of the surface opposed to this branch electrode is approximately Ld/2.
Furthermore, according to a third aspect of the invention, there is provided a light emitting diode of a double hetero-junction type in which a light-emitting layer made of a GaAlInP material is interposed between a p-type cladding layer and an n-type cladding layer, comprising:
a current blocking layer formed on a p-type cladding layer-side surface having an area of 0.15 mm2 or more; and
a p-side electrode formed at a position above the current blocking layer and opposed to the current blocking layer,
wherein any point present in a region not containing the current blocking layer of the p-type cladding layer-side surface is within a distance of (Ldxc3x972) from some point on an edge of the current blocking layer, where Ld is a distance from a position at which an optical power is maximum, to a position at which the optical power attenuates by 90%.
According to the construction, it is possible to obtain a favorable current diffusion and thus, suppress the increase of the current density. Therefore, even if the LED is used at a large electric current, the current density will not become too high. Accordingly, it is possible to prevent saturation of the optical power of the LED and deterioration in application of electric current to the LED. Thus, it is possible to improve the light-emitting characteristic at a large current.
Regarding the distance Ld, when the distribution of the optical output of an LED chip is examined along a line Bxe2x80x94Bxe2x80x2 passing through a p-side electrode 163, as shown in FIG. 19A, the distance Ld is a distance from a position close to a current blocking layer 164 at which an optical power is maximum, to a position at which the optical power attenuates by 90% as compared with the maximum optical power, as shown in FIG. 19B. Then, according to the present invention, as shown in an explanatory illustration of FIG. 20, the current blocking layer (denoted by 165 in FIG. 20) is provided such that any point present in a region not containing the current blocking layer of the p-type cladding layer-side surface is within the distance of (Ldxc3x972) from the edge of the current blocking layer.
In one embodiment, the current blocking layer comprises a plurality of blocking branch portions and a connection portion connecting the blocking branch portions to each other electrically, and an interval between adjacent blocking branch portions is approximately Ld.
The surface on which the current blocking layer may formed has two opposed parallel straight sides, and the blocking branch portions may be each strip-shaped and arranged parallel with the two sides and with each other.
In one embodiment, an interval between an outermost blocking branch portion and the side of the surface opposed to this outermost blocking branch portion is approximately Ld/2.
There is also provided, according to a fourth aspect of the present invention, a light emitting diode of a double hetero-junction type in which a light-emitting layer made of a GaAlInP material is interposed between a p-type cladding layer and an n-type cladding layer, wherein:
a p-side electrode is formed on a p-type cladding layer-side surface, the p-side electrode consisting of a plurality of mutually connected constituent parts; and
any point present in a region not containing the p-side electrode of the p-type cladding layer-side surface is within a distance of (Ldxc3x972) from some point on an edge of the p-side electrode, where Ld is a distance from a position at which an optical power is maximum, to a position at which the optical power attenuates by 90%.
In the LED according to any one of the second through the fourth aspects, if a current diffusion layer made of an AlGaInP material is provided between the p-type cladding layer and the p-side electrode, electric current favorably diffused by the p-side electrode or the current blocking layer is further diffused positively by the current diffusion layer. In this manner, it is possible to obtain more favorable current diffusion.
Also, if a barrier layer having a band gap intermediate between band gaps of the light-emitting layer and the p-type cladding layer is provided between the light-emitting layer and the p-type cladding layer, it is possible to prevent a p-type impurity from diffusing to the light-emitting layer, although the p-type impurity has a large impurity gradient and is liable to diffuse when electric current is passed through the LED for a long time. Thus it is possible to prevent deterioration of the optical output of the LED and improve the reliability thereof.
Furthermore, if a barrier layer having a band gap intermediate between band gaps of the light-emitting layer and the n-type cladding layer is provided between the light-emitting layer and the n-type cladding layer, it is possible to prevent an n-type impurity from diffusing to the light-emitting layer. Thus it is possible to prevent deterioration of the optical output power of the LED and improve the reliability thereof.
Other objects, features and advantages of the present invention will be obvious from the following description.